[sdiy] Fixing a dead Polysix voice

[Tom Wiltshire]

All the recent talk of polysixes made me decide to get round to fixing 
one of mine (I've got two, both in a poor state.)

The worst problem is that one of them has a dead voice. I've traced 
this to the VCO, since the other parts of the voice are tested and ok.

The VCO circuit is online at:

http://www.tomwiltshire.co.uk/sdiy/graphics/polysix/KorgPolysixVCO.gif

Testing the good voices gives me a healthy ramp from 0V to 4V at the 
'ramp' test point marked on the diagram. The dead voice shows 7V at 
this point. I also know that the CV inputs to all voices are good.

In general terms, this narrows it down to "something dead on the left 
hand side of the picture". I don't know how to work out in more detail 
exactly which part (one of the transistors? IC37?) or what's the most 
likely candidate.

Any ideas appreciated.


PS: The other polysix is in pretty bad mechanical shape, so I may well 
scrap it for spares. If anyone is interested in parts to keep their 
polysix going or to make it look better (spares keys, etc), contact me 
off list.

PPS: I also did a diagram of the filter, since I've wondered about 
building one as a stand alone:
http://www.tomwiltshire.co.uk/sdiy/graphics/polysix/PolysixSSM2044.gif


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